My son used to sleep in awful postitions and undo all the work he had done in therapy in the day. Meaning I'd have to wake and move him throughout the night. We were suggested a sleep system, this is a system which holds the child down into a fixed and comfortable position. We also had to raise one end of the bed which made gravity help rather than hinder. HOWEVER these only work for some children, but maybe worth a try / suggesting to therapists.
To pick up on another point about getting hot, I met an OT with CP the other day and she said that her body temprature is off most of the time and she can be at very strange tempratures throughout the day, my son does this too. Its worth watching as tone can be affected by being too hot or cold. The better their tone the more relaxed and the better they sleep, (thats the theory anyway!) Also just to back up the mattress comment - they make a MASSIVE difference, its worth paying a bit more if needed.